Multilayer wiring sheets are used conventionally in electronic computer circuitry. The operating speeds of computers are continuously increasing. To achieve a high operating speed, high speed signal transmission is needed. For high speed transmission, signal delay must be minimised at every part of the system, including the wiring assembly connecting to the integrated circuitry.
A typical layered wiring assembly has a ceramics substrate on which are laminated one or more insulating films. The insulating films support and separate conductive circuit layers, usually of copper. In conventional manufacture, insulator layers and patterned circuit layers are layered progressively onto the ceramics substrate, with each conductive layer being connected to the preceding one at the desired locations through through-holes of the insulator layers, at each stage of the process.
The number of electrical connections to be made in this sequential lamination process may be very large, typically thousands per layer. The best conventional process uses localised metal plating, typically electroless plating of copper, to plate through the through-holes (or "vias") and thereby connect an underlying conductor with a connection pad, adjacent the through-hole, of the upper conductor. The plating process involves many heat cycles and chemical treatment steps, and the resulting plated junction is liable both to damage and to peeling from the pad portions. It is therefore expensive, and the reliability problem makes it hard to achieve wiring assemblies having large numbers of layers e.g. more than ten.
Another conventional method uses a conductive solder paste whereof a lump is placed in the through-hole and fused by heating so that its top flows over the exposed conductor. The conductivity of these solder pastes is low, so they are inappropriate for high-speed signal transmission. They are also liable to cracking, and their adhesion to the conductors is not good.